1. Field of the Invention
The present invention relates to a tracking control apparatus for a video tape recorder, and more particularly, to a tracking control apparatus for a video tape recorder in which a head is controlled to precisely travel on predetermined tracks by using both the rotation number of a capstan motor and tracking errors detected by the head. A tracking control method is also disclosed.
2. Description of Related Art
In video tape recorders (VTRs), the purpose of tracking control is to obtain the maximum possible envelope output by controlling a video head to precisely travel on a prescribed track. During tracking control, if the head precedes a predetermined track, the speed of a capstan motor is controlled to become slower and therefore the travelling speed of tape becomes slower. If the head falls behind the predetermined track, the speed of the capstan motor is increased, thereby making tape travel faster.
A tracking error must be detected for such tracking control. A VHS-VTR records a control signal on a linear track. On the other hand, an 8 mm-VTR records a pilot signal on the data track so as to permit automatic track following (hereinafter referred to as ATF). In the 8 mm-VTR using the pilot signal, tracks on which video signals are recorded are divided using a prescribed number of pilot signals having different frequencies from one another. During playback, the tracking error is calculated using the detected pilot signals. A conventional apparatus for controlling the head to precisely travel on tracks by reducing tracking errors will now be discussed with reference to FIG. 1 through FIG. 3B.
FIG. 1 is a block diagram of a conventional tracking control apparatus. In FIG. 1, a tracking controller 21, receiving a tracking error signal TE from a tracking error signal generator (not shown), generates a first control signal C1 for controlling the speed of the capstan relevant to the speed of a head drum (not shown). A capstan motor 24 rotates the capstan for use in transporting tape. A capstan motor driver 23 generates a capstan motor driving signal. A capstan speed controller 22 receives a frequency signal (hereinafter referred to as FG signal) corresponding to the rotation number of capstan motor 24 and outputs a second control signal C2 for rotating the capstan at a predetermined speed by using the FG signal. An adder 25 adds the output signal C1 of tracking controller 21 and output signal C2 of capstan speed controller 22 and outputs the added result to capstan motor driver 23.
FIG. 2A illustrates diagonally disposed ideal tracks and head travel track H. FIG. 2B illustrates tracking error signal TE detected by head travel in the playback mode.
FIG. 3A illustrates tracks of the head linearly travelling on crooked tracks. FIG. 3B shows tracking errors detected from the similarly crooked tracks by the linearly travelling head. Normal tracks are in the form that is shown in FIG. 2A, that is, parallel to each other and diagonal to the tape. However, if the tracks are crooked as shown in FIG. 3A due to poor linearity, it is the crooked tracks which store information. When the information stored on those tracks is read, tracking errors are repeatedly generated having similar magnitudes, as depicted in FIG. 3B. Tracking control for the case in which the apparatus of FIG. 1 reads a signal from a video tape having such nonlinear tracks will be explained in greater detail immediately below.
Tracking controller 21 receives tracking error signal TE obtained from the pilot signal and generates first control signal C1. Capstan speed controller 22 calculates the rotation speed of capstan motor 24 by using the FG signal supplied from the capstan motor 24 and compares the calculated rotation speed and a predetermined rotation speed of capstan motor 24, thereby generating second control signal C2. Adder 25 adds above-mentioned control signals C1 and C2 and outputs a third control signal C3. Capstan motor 24 is driven by third control signal C3 applied to capstan motor driver 23. Tracking error signal TE and rotation number signal FG from capstan motor 24 continue to be used during tracking control. However, there is a slight time difference between the generation of a speed control signal C3 for head tracking with respect to tracking error signal TE and the FG signal of capstan 24, and the driving of the capstan motor 24 according to a generated speed control signal. Thus, the position on an actually controlled track falls behind a position corresponding to which a tracking error is detected. This prevents a precise tracking control, especially when tracks are severely deformed. It will be noted that the tracking control is more difficult to perform, resulting in poor displayed picture quality. Such a problem becomes more serious when a narrower track is used for high-density recording.
A technique for precise tracking control on crooked tracks is disclosed in U.S. Pat. No. 5,182,683, published on Jan. 26, 1993, by Mitsuhashi et al. In this patent, a signal read out from a track via a magnetic head is envelope-detected and converted into a digital envelope signal. A capstan is then controlled so that the intensity of the digital envelope signal reaches a substantially maximum value. Simultaneously, a prestored ideal scanning pattern of the magnetic head is compared with the digital envelope signal. On the basis of the compared result, the ideal scanning pattern is corrected, and the position of the magnetic head is changed in substantially perpendicular direction to the rotation axis of the rotating drum so that the intensity of the envelope signal becomes the substantial maximum value.
Another reference for controlling tap travelling is U.S. Pat. No. 4,581,659, published on Apr. 8, 1986, by Azuma et al. This disclosure teaches that a set value for control of a capstan motor is re-adjusted by using the differene between a present-track tracking error signal and a previous-track tracking error signal.